Printed circuit boards are widely used in modern electronic devices, wherein the circuit board typically comprises a substrate formed from an insulative material with conductive film applied thereto in a prescribed pattern or trace to provide conductive connections between electronic components, such as solid state chips, resistors, capacitors, etc. In this regard, the electronic components commonly include metal prongs adapted for reception into holes formed in the circuit board. The metal prongs are then attached to the circuit board by a soldering process which concurrently connects the components electrically to the conductive film on the board. This soldering step has been achieved by so-called wave soldering wherein the circuit board is conveyed over a standing wave of molten solder material to wet the metal prongs of the electronic components which protrude downwardly through the board. Alternatively, a so-called reflow solder process has been used wherein a solder-based paste is applied to the metal prongs of the electronic components mounted on the board and thereafter the resultant assembly is subjected to sufficient heat to reflow the solder to form the desired electrical connection.
In the past, according to a conventional wave-soldering process, circuit boards with unsoldered components in place are transported on a conveyor in succession to in-line fluxing, heating, and soldering stations. At the heating station, the circuit board is typically heated to approximately 100.degree. C. to activate and dry a flux material for purposes of cleaning each solder site by driving off volatiles which might spatter or otherwise interfere with the soldering step. Heating also minimizes thermal shock to the board during exposure of the board to a molten solder wave. Heating can also assist the solder flow into small holes in the circuit board, particularly such as multilayer circuit boards. If heating is not properly controlled, the risk of thermal damage to the board is increased (for example, cracks in the board may result) and reliability of the components can be reduced.
Control of the heating stage in a typical wave solder machine is often difficult. Such a machine operates typically with straight line conveyors which may be slightly inclined when passing over a molten solder wave, and which typically operate at the same speed regardless of different circuit board thickness and size. If more heating is required, as is especially the case with modern larger and thicker circuit boards, a requisite increased heat input is applied quickly to both the top and bottom sides of the board. In addition to being potentially damaging to the board and components, the heat distribution is often uneven causing problems upon contact with the solder wave.
Attempts have been made to address these heating problems by increasing conveyor lengths and correspondingly enlarging the heating station, or by slowing down the entire soldering process to accommodate longer residence times in the heating stage. Unfortunately, lengthened conveyors and enlarged heating stations occupy more floor space, which is often at a premium in manufacturing facilities. By contrast, slower conveyor speeds result in reduced solder production rates and correspondingly increased per unit cost.
The flux station in a solder process has also been the subject of considerable design focus, especially with respect to attempts to reduce or eliminate the need for typically toxic flux materials. In this regard, the flux material is normally applied to a solder site to remove oxide residue which can interfere with a reliable solder joint. Fluxes have been especially important in advance of heating stage, since oxidation tends to be enhanced in the presence of heat. However, it has been shown that if soldering takes place in an essentially oxygen-free cover-gas atmosphere (instead of in an air atmosphere as is usually the case today), the use of fluxing agents is minimized or eliminated. This process is known as inert-gas soldering. See, e.g., G. Schouten, No Fluxing, No Cleaning: Inert-Gas Wave Soldering, Circuits Manufacturing, September 1989. With this process, the molten solder and the solder joints no longer need to be protected against oxidation. Soldering becomes cleaner producing considerably less flux residue, if any, making post-solder solvent cleaning unnecessary. Soldering also becomes more reliable because the resultant oxide-free molten solder leads to fewer faulty joints with commensurate reduction in solder use and waste.
Accordingly, there has been a need for an improved soldering machine that has a heating station designed to provide and maintain an even heat distribution at selected maximum temperature, without requiring increased floor space or reduced process rates in a manufacturing facility. Moreover, there has existed a need for such an improved machine which is also designed to perform solder process steps in an essentially inert atmosphere, whereby the use of flux may be virtually eliminated. The present invention fulfills these needs and provides other related advantages.